law enforcement agencies are coming through evidence to try to find out who's behind the attack at the boston marathon. two bombs went off killing three people and injuring 176. the bombs went off on monday afternoon. the first knocked back runners who are 30 meters from the finish. the second hit people a short distance away. emergency medical crews rushed to the scene. the dead included an 8 year old boy. no one has claimed responsibility for the attack. 17 people were seriously wounded. authorities say they're carrying out a criminal probe. they say it's a potentially terrorist investigation. u.s. property barack obama says the bombings were an act of terrorism. >> they were the beginning of our investigation. it will take time to follow every lead and to determine what happened. but we will find out. we will find whoever harmed our citizens, and we will bring them to justice. >> obama says the government is on high alert for a new terror attack. he has called on security authorities to take necessary measures. >>> a japanese marathoner captures some scenes of the chaos and confusio